1) Field of the Invention
The present invention relates to a duplex transferring apparatus that transfers visual images on both sides of a recording medium, and an image forming apparatus using the duplex transferring apparatus.
2) Description of the Related Art
Conventionally, an image forming apparatus using a switchback method disclosed in Japanese Patent Application Laid-Open No. H5-8948 and an image forming apparatus using a one-path method disclosed in Japanese Patent Application Laid-Open No. H11-24443 are well known as a duplex-printing image forming apparatus that forms images on both sides of a recording medium. According to the switchback method, a recording medium is allowed to pass through a transfer unit and a fixing unit to record an image on one side thereof, and then the recording medium is reversed in a reversing carrier path. The recording medium is then switched back to the transfer unit and the fixing unit, to record an image on the other side thereof. On the other hand, the one-path method uses a duplex transferring apparatus that can continuously transfer visual images on the both sides of the recording medium. By having the recording medium passed through the duplex transferring apparatus to pass through the fixing unit, images can be recorded on the both sides of the recording medium, without switching back the recording medium.
In both methods, there is a problem such that a difference in the image quality between the front and back sides of the recording medium is conspicuous. Therefore, the present inventor has made extensive study for the cause of this problem, and has found the following points. That is, the transferability of the visual image is different according to the surface characteristics of the recording medium. For example, as the surface smoothness, which is one of the surface characteristics of the recording medium, deteriorates, the transferability of the visual image with respect to the recording medium deteriorates. Furthermore, in the transfer of a transfer charger method in which it is difficult to employ the known constant current control for controlling a transfer current flowing to the transfer unit to be constant, the transferability of the visual image with respect to the recording medium deteriorates, with a decrease in the surface electrical resistance, which is one of the surface characteristics of the recording medium. It is because electric charges applied by a transfer charger are hardly held on the surface of the recording medium, thereby making it difficult to form a transfer field of a desired intensity in the transfer unit. If the surface characteristics are different on the front and the back sides of the recording medium, the transferability with respect to the first side of the recording medium and the transferability with respect to the second side of the recording medium become different from each other. Nevertheless, if the visual image is transferred under the same transfer condition with respect to the first side and the second side of the recording medium, a difference in image quality occurs between the first and the second sides. For example, degradation of image caused by insufficient transfer occurs on the surface having poor surface smoothness, or electric charges are not held well on a surface having a smaller surface electrical resistance to cause degradation of image caused by insufficient transfer on the opposite surface, thus the difference in image quality occurs between the first and the second sides.
Therefore, the applicant of the present invention is now developing an image forming apparatus including a surface characteristic detector that detects the surface characteristics of the front and the back of the recording medium individually, and a transfer-condition setting unit that sets the transfer condition with respect to the first and the second sides individually based on the detection result. In this image forming apparatus, the first transfer condition at the time of transferring the first visual image onto the first side of the recording medium is set to a condition suitable for the surface characteristics of the first side by the transfer-condition setting unit. On the other hand, the second transfer condition at the time of transferring the second visual image onto the second side of the recording medium is set to a condition suitable for the surface characteristics of the second side. Accordingly, insufficient transfer is suppressed on both first and second sides, thereby enabling suppression of a difference in image quality between the first and the second sides due to the occurrence of degradation of image quality caused by insufficient transfer on either one surface of the recording medium.
Even in this image forming apparatus, however, a difference in image quality can occur between the first and the second sides. Therefore, the present inventor made intensive study for the cause why a difference in image quality cannot be suppressed reliably with the image forming apparatus, and has found the following points. That is, in the image forming apparatus, an optical sensor unit is used as the surface characteristic detector. This optical sensor unit has a first-side reflected-light detector that detects the optical reflectance on the first side by a reflecting type photosensor, and a second-side reflected-light detector that detects the optical reflectance on the second side by the reflecting type photosensor. The optical reflectance on the surface of the recording medium has a correlation with the surface smoothness. Therefore, by detecting the optical reflectance, the surface smoothness on the first and the second sides can be detected. However, the first-side reflected-light detector and the second-side reflected-light detector are arranged at positions at which a part of light emitted from one light-emitting element can be transmitted through the recording medium and detected by the other light-receiving element. Accordingly, interference occurs between the first and the second-side reflected-light detectors, and hence the surface smoothness on the first and the second sides cannot be accurately detected. Therefore, a difference in image quality occurs between the first and the second sides.
A problem that occurs in an example, in which the optical sensor unit that detects the surface smoothness on the first and the second sides of the recording medium individually by the optical method using the reflecting type photosensor is used as the surface characteristic detector, has been explained above. However, such a problem can occur not only in this example, but also when such a detector that causes interference is used as the first side characteristic detector that detects surface characteristics of the first side and the second side characteristic detector that detects surface characteristics of the second side is used.